Electronic devices containing active organic materials are attracting increasing attention for use in devices such as organic light emitting diodes (OLEDs), organic photoresponsive devices (in particular organic photovoltaic devices and organic photosensors), organic transistors and memory array devices. Devices containing active organic materials offer benefits such as low weight, low power consumption and flexibility. Moreover, use of soluble organic materials allows use of solution processing in device manufacture, for example inkjet printing or spin-coating.
An OLED may comprise a substrate carrying an anode, a cathode and one or more organic light-emitting layers between the anode and cathode.
Holes are injected into the device through the anode and electrons are injected through the cathode during operation of the device. Holes in the highest occupied molecular orbital (HOMO) and electrons in the lowest unoccupied molecular orbital (LUMO) of a light-emitting material combine to form an exciton that releases its energy as light.
A light emitting layer may comprise a semiconducting host material and a light-emitting dopant wherein energy is transferred from the host material to the light-emitting dopant. For example, J. Appl. Phys. 65, 3610, 1989 discloses a host material doped with a fluorescent light-emitting dopant (that is, a light-emitting material in which light is emitted via decay of a singlet exciton).
Phosphorescent dopants are also known (that is, a light-emitting dopant in which light is emitted via decay of a triplet exciton).
WO 2005/059921 discloses an organic light-emitting device comprising a hole-transporting layer and an electroluminescent layer comprising a host material and a phosphorescent material. High triplet energy level hole-transporting materials are disclosed in order to prevent quenching of phosphorescence.
WO 2010/119273 discloses an organic electroluminescent device having first and second electroluminescent layers including an electroluminescent layer comprising a hole-transporting material and an electroluminescent electron trapping material.
WO 2012/052713 discloses an organic light-emitting device having a charge-transporting layer doped with a light-emitting dopant and a light-emitting layer wherein the colour of light emitted from the device is wherein the x-coordinate value and/or the y-coordinate value of CIE(x,y) coordinates of light emitted from the device is no more than 0.1, and preferably no more than 0.05, from the respective x- or y-coordinate value of a control device in which the charge transporting layer is not doped with a light-emitting dopant.
WO 02/43447 discloses a device having a hole-transporting layer formed of a conjugated polymer and a layer consisting of an electroluminescent material. A buffer layer may be provided between the anode and the conjugated polymer layer.
WO 2008/024380 discloses a device having an anode; a buffer layer; a first hole transport layer; a second hole transport layer; and a photoactive layer comprising an anthracene derivative.
JP 2004/139819 discloses an organic electroluminescent device having a first hole-transporting layer, a second hole-transporting layer and a light-emitting layer containing a host and a phosphorescent dopant.